1. Field of the Invention
The present invention relates to an improved magnetic hard disc driving device (HDD) and, more particularly, to a very thin magnetic hard disc driving device.
2. Description of the Related Art
Presently, in accordance with a growing demand for enlarging a storage capacity of a magnetic disc device and in particular, of a magnetic hard disc driving device (HDD), there has arisen a growing demand for improving the flatness of a rotating hard disc for both recording and reproducing operations.
Generally, the magnetic disc is mounted on a hub which is fixed on a shaft of a spindle motor in a press-fitted manner or the like. The spindle motor, as used herein is referred to as an "in-hub type motor", of which the rotor and the stator are built in a concave portion of the hub which supports a disc. This type of motor is widely employed in a very thin hard disc driving device because the in-hub type motor can be hermetically assembled to a compact size.
In order to meet the demand for improving the flatness of the rotating magnetic disc, there are proposed many measures for flatly supporting the hard disc on the hub of the in-hub type motor in the prior arts.
Next, a description is given of an example of a disc driving device of the prior arts.
FIG. 1 is a cross sectional view showing an example of a disc driving device in the prier art.
A disc driving device 100 in the prior art employs an in-hub type motor 101 comprising a hub 102 as a rotor having a cylindrical concave 102a and a magnet 102b disposed on the inner wall of the concave 102a, a stator 103 having a starer core 103a and coils 103b provided on the starer core 103a which is assembled in the cylindrical concave 102a of the hub 102, a shaft 104 fixed to the hub 102 by using an adhesive or a press fit to be rotated together with the hub 102, a pair of bearings 105, 105 provided between the the shaft 104 and the starer 103, a clamber 106 for clamping a magnetic disc 108 or a plurality of discs (not shown) on the hub 102 and a screw 107 having an external thread for pressing the clamper 106 to the hub 102.
The magnetic disc 108 having a center hole 109 is installed in the hub 102 by causing the center hole 109 to be inserted with the hub 102, and is depressed against the hub 102 at a peripheral portion of center hole 109 thereof by means of the clamper 106 and the screw 107 in such a manner that the external thread of the screw 107 engages with an internal thread 104a provided in an upper end of the shaft 104 at a center axis thereof so as to be rotated together with the hub 102, wherein the flatness of the magnetic disc 108 increases in proportion to an increase of the downward pressure applied to the magnetic disc 108 by the screw 107.
However, if the shaft 104 is constructed in a cylindrical column , i.e., a straight shaft, the shaft 104 is apt to be driven out of the hub 102 by overcoming fixing force between the shaft 104 and the hub 102 and between the shaft 104 and the bearings 105, 105 when the screw 107 is tightened, so that it is impossible to fix securely the magnetic disc 108 on the hub 102 by the screw 107.
As a general countermeasure for preventing the shaft 104 from being driven out, the shaft is provided with a step portion 104b which prevents the screw 107 from causing a shearing force between the shaft 104 and bearings 105, 105 because the hub 102 as well as the clamper 106 pinched between the step portion 104b of the shaft 104 and the screw 107 as shown in FIG. 1, however, this poses problems of causing an increase a production cost of the shaft 104, and degrading the strength of the bearings 105, 105 because the shaft 104 has to be provided with the step portion 104b and each of the inner diameter of the bearings 105, 105 increases corresponding with the increase of the diameter of the shaft 104 for providing the step portion 104b thereon. For instance, a basic static load (Co) rating of a bearing having an inner diameter of 3 mm and an outer diameter of 8 mm is 19 kgf, on the other hand, a basic static load rating of a bearing having an inner diameter 4 mm and an outer diameter 8 mm is 13 kgf.
Next, the description is given to another prior art example of the disc driving device.
FIG. 2 is a cross sectional view showing another example of the disc driving device in the prior art.
Referring to FIG. 2, this disc driving device 110 employs an in-hub type motor 111 having a hub 112 integrally constructed with a shaft 112a, which eliminates a possibility of the shaft 112a being driven out of the hub 112.
Other components are constructed as in the foregoing prior art example of in FIG. 1 and in FIG. 2, with identical components to those in the described prior art examples for simplicity depicted by identical reference characters without detailed explanation thereof.
On the other hand, in the in-hub type motor 111, the shaft 112a is generally finished by machining, so that the dimensional accuracy and roundness of the shaft 112a are degraded compared with a shaft finished with a centerless polishing.
Generally, in a spindle motor of a hard disc device, a clearance if present in a bearing part causes an axial runout and radial runout of the shaft on which the hub together with the magnetic disc is mounted, this poses generation of data errors in writing and reading data on and from the magnetic disc. Therefore, the clearances between the shaft 112a and the inner races of the bearings 105, 105, and between the outer races of the bearings 105, 105 and a bearing housing such as a sleeve 113, which sleeve 113 is fixed to a stator frame 114, need to be avoided by using an adhesive and/or a press-fit assembling.
Therefore, in the assembly process of the in-hub type motor 111, the best way to compensate the clearance between the shaft 112a and the inner races of the bearings 105, 105 is to machine the disc mounting surface 112f and the disc fitting surface 112c of the hub 112 after the motor 111 is assembled. However, in practice, the shaft 112a is usually press-fitted to the inner races of the bearings 105, 105 without such an adaptive working, taking account of an increase of the product cost of the hub 112. In the press-fitting process of the shaft 112a having such poor dimensional accuracy, however, the lace surfaces of the inner races (ball rolling surface) of the bearings 105, 105 are deformed, which poses a problem of degrading None Repeatitive Run-Out (NERO) characteristic which is important to the hard disc driving device.